Multiplex communication between access points and hub

ABSTRACT

An access point for use in a wireless local area network (WLAN) includes a plurality of wireless communication units, which are adapted to exchange data with mobile stations by transmitting and receiving signals over the air on different, respective frequency channels of the WLAN. A physical layer interface in the access point is adapted to be coupled to a communication medium, so as to connect the plurality of wireless communication units to communicate with a hub over a single physical link of the communication medium.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 10/370,211, filed Feb. 18, 2003, which is assignedto the assignee of the present patent application and whose disclosureis incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to wirelesscommunications, and specifically to methods and devices for improvingthe performance of wireless local area networks.

BACKGROUND OF THE INVENTION

[0003] Wireless local area networks (WLANs) are gaining in popularity,and new wireless applications are being developed. The original WLANstandards, such as “Bluetooth” and IEEE 802.11, were designed to enablecommunications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEEworking groups have defined the 802.11a, 802.11b and 802.11g extensionsto the original standard, in order to enable higher data rates. The802.11a standard, for example, envisions data rates up to 54 Mbps overshort distances in a 5 GHz band, while 802.11b defines data rates up to22 Mbps in the 2.4 GHz band. In the context of the present patentapplication and in the claims, the term “802.11” is used to refercollectively to the original IEEE 802.11 standard and all its variantsand extensions, unless specifically noted otherwise.

[0004] The theoretical capability of new WLAN technologies to offer highcommunication bandwidth to mobile users is severely hampered by thepractical limitations of wireless communications. Indoor propagation ofradio frequencies is not isotropic, because radio waves are influencedby building layout and furnishings. Therefore, even when wireless accesspoints are carefully positioned throughout a building, some “blackholes” generally remain—areas with little or no radio reception.Furthermore, 802.11 wireless links can operate at full speed only underconditions of high signal/noise ratio. Signal strength scales inverselywith the distance of the mobile station from its access point, andtherefore so does communication speed. A single mobile station with poorreception due to distance or radio propagation problems can slow downWLAN access for all other users in its basic service set (BSS—the groupof mobile stations communicating with the same access point in aconventional 802.11 WLAN).

[0005] The natural response to these practical difficulties would be todistribute a greater number of access points within the area to beserved. If a receiver receives signals simultaneously from two sourcesof similar strength on the same frequency channel, however, it isgenerally unable to decipher either signal. The 802.11 standard providesa mechanism for collision avoidance based on clear channel assessment(CCA), which requires a station to refrain from transmitting when itsenses other transmissions on its frequency channel. In practice, thismechanism is of limited utility and can place a heavy burden ondifferent BSSs operating on the same frequency channel.

[0006] Therefore, in high data-rate 802.11 WLANs known in the art,access points in mutual proximity must use different frequency channels.Theoretically, the 802.11b and 802.11g standards define 14 frequencychannels in the 2.4 GHz band, but because of bandwidth and regulatorylimitations, WLANs operating according to these standards in the UnitedStates actually have only three non-overlapping frequency channels fromwhich to choose. In the 5 GHz band, a larger number of frequencychannels is available.

SUMMARY OF THE INVENTION

[0007] Embodiments of the present invention provide access points foruse in a wireless local area network (WLAN), which are capable ofcommunicating on multiple frequency channels simultaneously, unlikeaccess points known in the art. Each such access point comprisesmultiple wireless communication units, each comprising its own radiotransceiver. Each transceiver is tuned for operation on a different,respective frequency channel of the WLAN. Therefore, from the point ofview of the mobile stations, each multi-channel access point behaveseffectively as though it were a set of several collocated single-channelaccess points. The transceivers in each access point may simultaneouslyserve multiple mobile stations, each on a respective channel, whileavoiding the need to deploy separate access points for each channel.

[0008] The access points communicate with a hub over a communicationmedium, typically a wired LAN. This medium serves, inter alia, as thedistribution system medium (as defined in the 802.11 specification) forconnecting the access points (APs) to networks external to the WLANsystem. The wireless communication units in each multi-channel accesspoint share a common physical layer interface (SHY) to the communicationmedium. A multiplexer, coupled between the channel processors and thephysical layer interface, enables all the wireless communication unitsto send and receive data over the same physical link of thecommunication medium. Since only a single physical link is thus requiredbetween each multi-channel access point and the hub, the cost andlogistics involved in deploying the multi-channel access points are nogreater than would be required for conventional, single-channel accesspoints.

[0009] In some embodiments of the present invention, the multiplexercombines the data from multiple channels into data frames fortransmission over the communication medium using a novel multiplexingprotocol. Each frame is divided into a sequence of slots, or chunks. Ineach chunk, the multiplexer inserts data from one of the channels andadds a chunk header indicating the channel to which the chunk belongs.The frames are demultiplexed and processed by the hub. The hub likewisetransmits multiplexed data frames over the communication medium to bedemultiplexed at each of the access points. The chunks may be used bothto carry data, which are transmitted by the access points to and fromthe mobile stations that they serve, and to carry control messagesbetween the hub and the access points themselves.

[0010] There is therefore provided, in accordance with an embodiment ofthe present invention, an access point for use in a wireless local areanetwork (WLAN), the access point including:

[0011] a plurality of wireless communication units, which are adapted toexchange data with mobile stations by transmitting and receiving signalsover the air on different, respective frequency channels of the WLAN;and

[0012] a physical layer interface, which is adapted to be coupled to acommunication medium, so as to connect the plurality of wirelesscommunication units to communicate with a hub over a single physicallink of the communication medium.

[0013] In some embodiments, the wireless communication units are adaptedto communicate with the mobile stations substantially in accordance withan IEEE 802.11 specification, which defines the frequency channels.

[0014] Typically, the communication medium includes a wired local areanetwork (LAN). In a disclosed embodiment, the physical layer interfaceis adapted to transmit and receive data frames over the communicationmedium in accordance with an Ethernet physical layer specification. Thecommunication medium may be a distribution system medium of the WLAN.

[0015] In some embodiments, the access point includes a multiplexer,coupled between the wireless communication units and the physical layerinterface so as to selectively convey the data from the plurality of thewireless communication units to the physical layer interface fortransmission over the single physical link. Typically, the access pointalso includes a demultiplexer, coupled between the wirelesscommunication units and the physical layer interface so as to distributethe data received over the single physical link among the plurality ofthe wireless communication units. The multiplexer and the demultiplexermay be adapted to convey control messages, in addition to the data,which are transmitted over the communication medium between the accesspoints and the hub.

[0016] Additionally or alternatively, the multiplexer is adapted togenerate frames of the data for transmission over the physical link, andto combine chunks of the data from two or more of the wirelesscommunication units into each of at least some of the frames. In adisclosed embodiment, each of the frames includes a plurality of slotsof substantially fixed length, and the multiplexer is adapted to insertthe chunks into respective slots together with chunk headers identifyingthe respective frequency channels to which the chunks belong. Themultiplexer may be adapted to transfer the chunks of the data from thetwo or more of the wireless communication units in alternation into eachof the at least some of the frames. In some embodiments, the dataconveyed from the plurality of the wireless communication units includesdata packets, and the multiplexer is adapted to fragment the packetsamong the chunks. Additionally or alternatively, the multiplexer isadapted to insert into the frames, in addition to the data, controlmessages for transmission between the access points and the hub.

[0017] There is also provided, in accordance with an embodiment of thepresent invention, a system for mobile communication, including:

[0018] a hub;

[0019] a communication medium, coupled to the hub; and

[0020] a plurality of access points, each of which includes:

[0021] two or more wireless communication units, which are adapted toexchange data with mobile stations by transmitting and receiving signalsover the air on different, respective frequency channels of a wirelesslocal area network (WLAN); and

[0022] a single physical layer interface, coupled to the communicationmedium, so as to connect the two or more wireless communication units tocommunicate with the hub over the communication medium.

[0023] Typically, the communication medium includes a plurality oflinks, which are coupled to the hub, and the physical layer interface iscoupled to a single, respective link among the plurality of the links ofthe communication medium.

[0024] In a disclosed embodiment, the access points have respectiveservice areas and are arranged so that at least some of the serviceareas substantially overlap. In this embodiment, the hub and thewireless communication units are arranged to exchange control messagesover the communication medium, via the single physical layer interface,so as to determine which of the wireless communication units is to serveeach of the mobile stations.

[0025] There is additionally provided, in accordance with an embodimentof the present invention, a method for mobile communication, including:

[0026] arranging an access point in a wireless local area network(WLAN), the access point including two or more wireless communicationunits, which are adapted to exchange data with mobile stations bytransmitting and receiving signals over the air on different, respectivefrequency channels of the WLAN;

[0027] coupling the access point to a hub over a single physicalcommunication link; and

[0028] conveying the data between the plurality of the wirelesscommunication units and the hub over the single link.

[0029] The present invention will be more fully understood from thefollowing detailed description of the embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a block diagram that schematically illustrates awireless LAN (WLAN) system, in accordance with an embodiment of thepresent invention;

[0031]FIG. 2 is a block diagram that schematically shows details of amulti-channel WLAN access point, in accordance with an embodiment of thepresent invention;

[0032]FIG. 3 is a block diagram that schematically illustrates amultiplexed data frame, in accordance with an embodiment of the presentinvention; and

[0033]FIG. 4 is a block diagram that schematically shows details of ahub in a WLAN system, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0034]FIG. 1 is a block diagram that schematically illustrates awireless LAN (WLAN) system 20, in accordance with an embodiment of thepresent invention. System 20 comprises multiple access points 22, whichare configured for data communication with mobile stations 24. Eachaccess point is capable of transmitting and receiving signalssimultaneously on multiple different frequency channels that areavailable on the WLAN, referred to as channels (CH) 0, 1, 2, 3, etc. (Bycontrast, in WLAN systems known in the art, each access point has asingle frequency channel.) Mobile stations 24 typically comprisecomputing devices, such as desktop, portable or handheld devices.

[0035] In the exemplary embodiment described hereinbelow, it is assumedthat the access points and mobile stations communicate with one anotherin accordance with one of the standards in the IEEE 802.11 family andobserve the 802.11 medium access control (MAC) layer conventions.Details of the 802.11 MAC layer are described in ANSI/IEEE Standard801.11 (1999 Edition), 1 and specifically in Part 11: Wireless LANMedium Access Control (MAC) and Physical Layer (PHY) Specifications,which is incorporated herein by reference. The principles of the presentinvention, however, are not limited to the 802.11 standards, and maylikewise be applied to substantially any type of WLAN, includingHiperLAN, Bluetooth and hiswan-based systems.

[0036] Each access points 22 is connected to a hub 26 by a link of awired LAN 28. LAN 28 is typically physically configured as an EthernetLAN, such as a 100BASE-TX LAN-, and serves as the distribution systemmedium (DSM), as defined in the 802.11 specification, for carrying datato and from mobile stations 24. This arrangement enables mobile stations24 to send and receive data through access points 22 to and from anexternal network 30, such as the Internet, via an access line 32connected to hub 26. The access points may also use LAN 28 tocommunicate with the hub and/or with one another in order to coordinatetheir use of the available frequency channels and responses to mobilestations.

[0037] Although LAN 28 may conform to standard physical layerspecifications, such as those provided by the Ethernet standard,however, access points 22 and hub 26 communicate over the LAN using anovel MAC-level multiplexing protocol. This protocol is described indetail hereinbelow. Alternatively or additionally, the access points andhub may use this novel protocol to communicate with one another oversubstantially any suitable communication medium, including wire,fiberoptics, or even free-space optical or radio communications (in anallowed frequency band that does not interfere with WLAN operation).

[0038] Access points 22 in system 20 are typically closely spaced, sothat radio waves in a given frequency channel may reach mobile station24 from multiple access points simultaneously, and radio messagestransmitted by the mobile station may be received at multiple accesspoints. In WLAN systems known in the art, under these circumstances,mobile station 24 would receive downlink messages from two or more ofthe access points, which would probably result in inability of themobile station to communicate with any of the access points. In someembodiments of the present invention, the access points collaborate toresolve this conflict by exchanging control messages with hub over LAN28 (or over whatever other medium is used to connect the access points).An exemplary method for such messaging and control is described in aU.S. patent application entitled, “Wireless LAN with Central Managementof Access Points,” filed Sep. 19, 2003. Alternatively, access points 22may be configured to arbitrate among themselves to determine whichaccess point is to serve a given mobile station, as described, forexample, in U.S. patent application Ser. No. 10/214,271, filed Aug. 7,2002; in U.S. patent application Ser. No. 10/272,686, filed Oct. 17,2002; or in U.S. patent application Ser. No. 10/348,863, filed Jan. 22,2003. All of the above applications are assigned to the assignee of thepresent patent application, and their disclosures are incorporatedherein by reference.

[0039] Alternatively, access points 22 may be deployed in a conventionalmanner, so that the coverage areas of different access points in system20 do not overlap. In this case, the special messaging and controlcapabilities described above are not required. Access points 22 arestill advantageous over access points known in the art, in that theyoperate on multiple frequency channels (and thus can serve a largernumber of mobile stations), while still using only a single link on LAN28 to communicate with hub 26.

[0040]FIG. 2 is a block diagram that schematically shows details ofaccess points 22, in accordance with an embodiment of the presentinvention. Each access point comprises multiple wireless communicationunits 42, each of which is configured to transmit and receive signalsvia an antenna 44 on a respective frequency channel of the WLAN.Accordingly, units 42 are labeled by channel: “channel 0,” “channel 1,”etc. Although access point 22 is shown in this figure as comprising foursuch wireless communication units, the access point may alternatively beconfigured to comprise a larger or smaller number of units, and thus mayaccommodate a larger or smaller number of frequency channels.

[0041] Each wireless communication unit 42 comprises a WLAN physicallayer interface (WLAN PHY) 50, comprising a radio transceiver that istuned to the respective frequency channel of the transceiver. WLAN PHY50 may comprise a standard, off-shelf device, such as the RD0314 board,made by RF Micro Devices (Greensboro, N.C.). The WLAN PHY devices areconnected to antenna 44 (or to a pair of antennas—not shown—fordiversity purposes) via an antenna multiplexing circuit 46. In anexemplary embodiment, circuit 46 is constructed as described in theabove-mentioned U.S. patent application Ser. No. 10/370,211.

[0042] A medium access control (MAC) processor 48 in each channelperforms higher-level message processing functions. Processor 48performs MAC-level processing of the uplink packets received by PHY 50from mobile stations 24, and generates downlink packets for transmissionby PHY 50, in accordance with the 802.11 standard (or any otherapplicable WLAN standard). In addition, processor 48 may be responsiblefor messaging over LAN 28, as described above, to determine which ofaccess points 22 is to serve each mobile station 24. Typically,processor 48 comprises a programmable microprocessor or logic device,such as a field-programmable gate array (FPGA), which is configured tocommunicate with WLAN PHY 50 and with LAN 28 (via a suitable LAN PHY, asdescribed below), and is programmed to carry out the functions describedherein.

[0043] MAC processors 48 in units 42 are linked to LAN 28 by a singleLAN physical layer interface (LAN PHY) 52, typically an Ethernet PHYdevice, such as the KS8721B Physical Layer Transceiver, made byMicrel-Kendin (Sunnyvale, Calif.). Each data frame transmitted from hub26 over LAN 28 may contain chunks of data belonging to multiplechannels, i.e., data that are destined for different units 42 in a givenaccess point 22. An exemplary format of these data frames is shown belowin FIG. 3. A receive demultiplexer 54 parses each of these frames so asto distribute the data chunks they contain to processors 48 in theappropriate units 42. Similarly, data chunks generated by processors 48for transmission over LAN 28 to hub 26 are multiplexed into data framesof this sort by a transmit multiplexer 56. Hub 26 performs comparablemultiplexing and demultiplexing functions for each access point, asdescribed below with reference to FIG. 4.

[0044] The functional blocks of access point 22 shown in FIG. 2 arechosen for conceptual clarity, and do not necessarily represent thephysical components that might actually be used to implement the designshown here. The functional blocks shown in the figure may be combinedinto one or more custom integrated circuit components, or they mayalternatively be broken into a larger number of custom or off-shelfcomponents. Logical and control elements of access point 22 may compriseeither hard-wired or programmable components, with appropriate software,as will be apparent to those skilled in the art.

[0045]FIG. 3 is a block diagram that schematically illustrates amultiplex data frame 60, which is transmitted over LAN 28, in accordancewith an embodiment of the present invention. Frames of this sort areassembled by transmit multiplexer 56 for transmission over LAN 28 to hub26, and are also received from the hub and disassembled by receivedemultiplexer 54. Frame 60 comprises a preamble 62, followed by asequence of data chunks 64 of fixed size. The preamble typicallycomprises a predefined bit sequence marking the beginning of the frame,for the purpose of synchronization of LAN PHY 52. For example, assumingLAN 28 to operate in accordance with the Ethernet 100BASE-Tspecification, preamble 62 is 01010101. Frame 60 typically contains afixed number of chunks 64, which may be any number up to a maximum thatis determined by the maximum permitted frame size on LAN 28.Alternatively, the sizes of frames 60 may be variable, up to thepermitted maximum.

[0046] Note that frame 60 typically does not include a conventional MACheader. Since hub 26 is connected to each access point 22 by adedicated, point-to-point link, all frames transmitted over the link arereceived by the appropriate receive demultiplexer. There is thus no needfor a MAC header to identify the source and destination addresses offrame 60 and other frame parameters. Alternatively, a MAC header may beadded to frame 60 if desired, with appropriate changes to the accesspoints and hub in order to handle the header.

[0047] Each chunk 64 comprises a chunk header (CHDR) 66 and a chunkpayload 68. The header comprises a code indicating the channel to whichthe chunk belongs and, optionally, additional control information. TableI below provides an exemplary specification of a four-bit chunk header,for use in embodiments in which access points 22 serve up to fourchannels. Payload 68 typically comprise a short data segment, forexample, forty bits of data, provided by or directed to processor 48 forthe channel in question. The data segment may comprise either a part ofa data packet conveyed via access point 22 between mobile station 24 andhub 26, or a control message exchanged between the access point and thehub. The control messages are used for system management functions, suchas determining which access point is to serve a given mobile station, asdescribed above. TABLE I CHUNK HEADER DEFINITION Header Meaning 1100Void chunk, no data to transmit cc11 First normal-priority chunk forchannel “cc” cc01 Non-first normal-priority chunk for channel “cc” cc10High-priority chunk for channel “cc”

[0048] High-priority chunks may optionally be used to carry special,short control messages that must be transmitted with low latency.Normal-priority chunks are used for data packets and for low-prioritycontrol messages. Alternatively, all chunks may have the same priority(in which case header cc10 is not used for this purpose).

[0049] When processor 48 on any channel has data—either a data packet ora control message—to send to hub 26, it signals multiplexer 56,typically by raising a “chunk_ready” flag. (Separate flags may beprovided for normal and high-priority messages.) The flag remains raisedas long as the processor has more data to send. Multiplexer 56 reads afixed length of data (forty bits in the present example) in turn fromeach processor that has raised its chunk_ready flag, while skipping overchannels that have no data. Any suitable multiplexing algorithm may beused for this purpose. For example, a round robin algorithm may be usedto serve all channels with equal priority. Alternatively, if one of theWLAN channels has greater capacity or higher priority than others, aweighted queuing algorithm may be used in order to increase the share ofthe bandwidth on LAN 28 that is allocated to that channel at the expenseof the others. When no channel has data to send, multiplexer 56 sends achunk of void data, which is ignored by hub 26.

[0050] Multiplexer 56 adds the appropriate chunk header 66 to eachchunk, depending on the originating channel and the priority of thechunk payload. Each data packet sent by processor 48 is typicallyfragmented among multiple chunks 64. Multiplexer 56 typically marks thefirst chunk of a given data packet with a special first chunk header, asshown in Table I, to indicate to the receiving side that a new packet isstarting. The first few chunks of any given packet contain the packetheader, followed by the packet payroll in subsequent chunks, and endinggenerally with an error detection code, such as a CRC, in the finalchunk or chunks.

[0051] Since the packet length is generally not an integer multiple ofthe chunk payroll size, processor 48 or multiplexer 56 may pad the lastchunk in the packet with dummy bits. As the packet header generallycontains a field that indicates the packet length, there is no need tomark the last chunk. Rather, the receiving processor identifies the lastchunk based on the known packet length and thus discards any paddingbits that have been added. Alternatively, the packets (or at leastcertain, predefined types of packets) may be of fixed size, in whichcase the receiving processor determines the packet length based on thepacket type. Further alternatively, the last chunk in each packet may bemarked with a special chunk header, instead of or in addition to markingthe first chunk.

[0052] After sending the last chunk 64 in a given frame 60, multiplexer56 waits for a predefined interval, typically the Inter-Packet Gap (IPG)interval provided by the physical layer specification of LAN 28. Themultiplexer then starts transmitting the next frame, beginning withpreamble 62.

[0053] Upon receiving frame 60 from hub 26, receive demultiplexer 54detects and removes preamble 62, and then reads chunk header 66 of thefirst chunk 64 in the frame to determine the channel for which the chunkis destined. The demultiplexer signals processor 48 of the appropriatechannel, typically by raising a write enable flag, and then passespayload 68 of the chunk to the processor after stripping header 66.Based on the chunk header, the demultiplexer may also signal theprocessor, as appropriate, to indicate that the current chunk is thefirst chunk in a packet or that the current chunk contains a controlmessage. Processor 48 reassembles data packets from the chunk fragmentsthat it receives and transmits the data packets to the appropriatemobile station 24.

[0054] Although the generation and demultiplexing of frames 60 isdescribed hereinabove with reference to access points 22, hub 26 alsoperforms frame generation and demultiplexing functions in asubstantially identical manner.

[0055]FIG. 4 is a block diagram that schematically shows details of hub26, in accordance with an embodiment of the present invention. Hub 26comprises multiple access point (AP) interface units 72, each of whichis connected via a link of LAN 28 to a respective access point 22. Inthe present example, the hub comprises eight such interfaces (for accesspoints AP1 through AP8), but the hub may alternatively serve a larger orsmaller number of access points.

[0056] AP interface units 72 are connected via multiplexing circuits, asdescribed hereinbelow, to channel processors 70. Each channel processor70 is assigned to process data and control messages for a respectivefrequency channel of the access points. In other words, the “channel 0processor” shown in FIG. 4 processes downlink and uplink messages to andfrom mobile stations that are transmitted and received by the “channel0” units 42 in all of access points 22, as well as sending and receivingcontrol messages to and from these units. Thus, four channel processors70 are shown in FIG. 4, corresponding to the four channels served by therespective wireless communication unit in each access point. Thefunctions of the channel processors (and possibly some of the functionsof AP interface units 72) may be performed by suitably-programmed logicdevices, such as the Cyclone™ EP1C20 FPGA chip, produced by Altera Corp.(San Jose, Calif.).

[0057] A central processing unit (CPU) 74 controls the operation ofchannel processors 70, as well as performing high-level messageprocessing functions. Although a single CPU is shown In FIG. 4 ascontrolling all the channel processors, in an alternative embodimenteach channel may have its own CPU, with connections between the channelCPUs for inter-channel signals. CPU 74 may comprise one or more IntelXScale™ processors, with an Ethernet reduced medium independentinterface (RMII) for communicating with the channel processors.

[0058] Frames transmitted over LAN 28 by each access point 22 arereceived by a LAN physical layer interface (LAN PHY) 76 in therespective AP interface unit 72. These frames have the form of frame 60,as shown in FIG. 3. Chunks 64 in these frames are-demultiplexed tochannel processors 70 by a receive demultiplexer 78, in the mannerdescribed above. Each channel processor comprises multiple port decoders80, each assigned to receive the chunk data from the receivedemultiplexer of the respective AP interface unit 72. The port decodersreassemble the uplink data packets from the fragments contained inchunks 64, and pass the packets to a receive controller 82. The receivecontroller performs basic MAC level packet processing functions, andthen places the packets in a dual-port RAM (DPRAM) 84. Although forconceptual clarity, decoders 80 and controller 82 are shown as separateunits, the functions of these units may be integrated in a singleprocessing stage, and the controller may thus be aware of the operationof demultiplexer 78.

[0059] A CPU interface controller 86 passes the packets from DPRAM 84 ineach of channel processors 70 to CPU 74. Depending on the destinationaddress of each uplink packet, the CPU may either route the packetdownlink via the appropriate channel and access point 22 to anothermobile station 24, or may route the packet via a network interface 88 toexternal network 30.

[0060] CPU 74 passes downlink packets (whether received from network 30via interface 88 or from mobile stations 24) via interface controller 86to another DPRAM 89. A transmit controller 90 reads the packets fromDPRAM 89, performs basic MAC level processing functions, and passes thepackets to a port multiplexer 92, which distributes the packets to theappropriate AP interface units 72. Multiplexer 92 sends each packet tothe AP interface unit that is associated with the access point that isassigned to serve the mobile station to which the packet is destined.Broadcast messages are distributed to all AP interfaces. In this manner,each AP interface unit 72 may receive inputs from all of channelprocessors 70. A transmit multiplexer 94 in the interface unitmultiplexes the downlink packets and control messages via LAN PHY 76onto LAN 28 in the manner described above.

[0061] In distinction to its treatment of uplink data packets, receivecontroller 82 passes control messages from access points 22 directly toCPU interface 86. Similarly, the CPU interface passes control messagesfor the access points directly to transmit controller 90. For certaintypes of signaling, receive controller 82 may also communicate directlywith transmit controller 90. For example, when an uplink message fromone of mobile stations 24 requires that the receiving access pointreturn an acknowledgment to the mobile station (as required by the802.11 specification), the receive controller may directly instruct thetransmit controller to signal the proper access point to issue theacknowledgment. In this manner, the latency of the acknowledgment issubstantially reduced.

[0062] As noted earlier, although the embodiments described above relateto certain specific communication protocols and hardware configurationsconforming with these protocols, the principles of the present inventionmay be applied to WLAN systems, access points and hubs of other types,operating in accordance with other protocols that are or may becomeknown in the art. It will thus be appreciated that the embodimentsdescribed above are cited by way of example, and that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and subcombinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofwhich would occur to persons skilled in the art upon reading theforegoing description and which are not disclosed in the prior art.

1. An access point for use in a wireless local area network (WLAN), theaccess point comprising: a plurality of wireless communication units,which are adapted to exchange data with mobile stations by transmittingand receiving signals over the air on different, respective frequencychannels of the WLAN; and a physical layer interface, which is adaptedto be coupled to a communication medium, so as to connect the pluralityof wireless communication units to communicate with a hub over a singlephysical link of the communication medium.
 2. The access point accordingto claim 1, wherein the wireless communication units are adapted tocommunicate with the mobile stations substantially in accordance with anIEEE 802.11 specification, which defines the frequency channels.
 3. Theaccess point according to claim 1, wherein the communication mediumcomprises a wired local area network (LAN).
 4. The access pointaccording to claim 3, wherein the physical layer interface is adapted totransmit and receive data frames over the communication medium inaccordance with an Ethernet physical layer specification.
 5. The accesspoint according to claim 1, wherein the communication medium is adistribution system medium of the WLAN.
 6. The access point according toclaim 1, and comprising a multiplexer, coupled between the wirelesscommunication units and the physical layer interface so as toselectively convey the data from the plurality of the wirelesscommunication units to the physical layer interface for transmissionover the single physical link.
 7. The access point according to claim 6,and comprising a demultiplexer, coupled between the wirelesscommunication units and the physical layer interface so as to distributethe data received over the single physical link among the plurality ofthe wireless communication units.
 8. The access point according to claim7, wherein the multiplexer and the demultiplexer are adapted to conveycontrol messages, in addition to the data, which are transmitted overthe communication medium between the access points and the hub.
 9. Theaccess point according to claim 6, wherein the multiplexer is adapted togenerate frames of the data for transmission over the physical link, andto combine chunks of the data from two or more of the wirelesscommunication units into each of at least some of the frames.
 10. Theaccess point according to claim 9, wherein each of the frames comprisesa plurality of slots of substantially fixed length, and wherein themultiplexer is adapted to insert the chunks into respective slotstogether with chunk headers identifying the respective frequencychannels to which the chunks belong.
 11. The access point according toclaim 9, wherein the multiplexer is adapted to transfer the chunks ofthe data from the two or more of the wireless communication units inalternation into each of the at least some of the frames.
 12. The accesspoint according to claim 9, wherein the data conveyed from the pluralityof the wireless communication units comprises data packets, and whereinthe multiplexer is adapted to fragment the packets among the chunks. 13.The access point according to claim 9, wherein the multiplexer isadapted to insert into the frames, in addition to the data, controlmessages for transmission between the access points and the hub.
 14. Asystem for mobile communication, comprising: a hub; a communicationmedium, coupled to the hub; and a plurality of access points, each ofwhich comprises: two or more wireless communication units, which areadapted to exchange data with mobile stations by transmitting andreceiving signals over the air on different, respective frequencychannels of a wireless local area network (WLAN); and a single physicallayer interface, coupled to the communication medium, so as to connectthe two or more wireless communication units to communicate with the hubover the communication medium.
 15. The system according to claim 14,wherein the communication medium comprises a plurality of links, whichare coupled to the hub, and wherein the physical layer interface iscoupled to a single, respective link among the plurality of the links ofthe communication medium.
 16. The system according to claim 14, whereinthe access points have respective service areas and are arranged so thatat least some of the service areas substantially overlap.
 17. The systemaccording to claim 16, wherein the hub and the wireless communicationunits are arranged to exchange control messages over the communicationmedium, via the single physical layer interface, so as to determinewhich of the wireless communication units is to serve each of the mobilestations.
 18. The system according to claim 14, wherein the wirelesscommunication units are adapted to communicate with the mobile stationssubstantially in accordance with an IEEE 802.11 specification, whichdefines the frequency channels.
 19. The system according to claim 14,wherein the communication medium comprises a wired local area network(LAN).
 20. The system according to claim 19, wherein the physical layerinterface is adapted to transmit and receive data frames over thecommunication medium in accordance with an Ethernet physical layerspecification.
 21. The system according to claim 14, wherein thecommunication medium is a distribution system medium of the WLAN. 22.The system according to claim 14, wherein each of the access pointscomprises a multiplexer, coupled between the wireless communicationunits and the physical layer interface so as to selectively convey thedata from the plurality of the wireless communication units to thephysical layer interface for transmission over the communication medium.23. The system according to claim 22, wherein each of the access pointscomprises a demultiplexer, coupled between the wireless communicationunits and the physical layer interface so as to distribute the datareceived over the communication medium among the plurality of thewireless communication units.
 24. The system according to claim 23,wherein the multiplexer and the demultiplexer are adapted to conveycontrol messages, in addition to the data, which are transmitted overthe communication medium between the hub and the access points.
 25. Thesystem according to claim 22, wherein the multiplexer is adapted togenerate frames of the data for transmission over the communicationmedium, and to combine chunks of the data from two or more of thewireless communication units into each of at least some of the frames.26. A method for mobile communication, comprising: arranging an accesspoint in a wireless local area network (WLAN), the access pointcomprising two or more wireless communication units, which are adaptedto exchange data with mobile stations by transmitting and receivingsignals over the air on different, respective frequency channels of theWLAN; coupling the access point to a hub over a single physicalcommunication link; and conveying the data between the plurality of thewireless communication units and the hub over the single link.
 27. Themethod according to claim 26, wherein arranging the access pointcomprises arranging multiple access points in the WLAN, and whereincoupling the access point comprises coupling each of the access pointsto communicate with the hub over a single, respective link amongmultiple links provided by a communication medium that is connected tothe hub.
 28. The method according to claim 27, wherein the access pointshave respective service areas, and wherein arranging the multiple accesspoints comprises arranging the access points so that at least some ofthe service areas substantially overlap.
 29. The method according toclaim 28, and comprising exchanging control messages between the accesspoint and the hub over the single, respective link, so as to determinewhich of the wireless communication units is to serve each of the mobilestations.
 30. The method according to claim 26, wherein arranging theaccess point comprises configuring the wireless communication units tocommunicate with the mobile stations substantially in accordance with anIEEE 802.11 specification, which defines the frequency channels.
 31. Themethod according to claim 26, wherein coupling the access point to thehub comprises coupling the access point and the hub to a wired localarea network (LAN).
 32. The method according to claim 31, whereinconveying the data comprises transmitting and receiving data frames overthe LAN in accordance with an Ethernet physical layer specification. 33.The method according to claim 31, wherein the LAN is a distributionsystem medium of the WLAN.
 34. The method according to claim 26, whereinconveying the data comprises multiplexing between the wirelesscommunication units and the single physical link so as to selectivelyconvey the data from the plurality of the wireless communication unitsto the single physical link for transmission to the hub.
 35. The methodaccording to claim 34, wherein conveying the data further comprisesdemultiplexing the data received over the single physical link from thehub for distribution among the plurality of the wireless communicationunits.
 36. The method according to claim 35, and comprising transmittingcontrol messages over the single physical link between the hub and theaccess point, and multiplexing and demultiplexing the control messagesamong the plurality of the wireless communication units.
 37. The methodaccording to claim 34, wherein multiplexing between the wirelesscommunication units comprises generating frames of the data fortransmission over the communication medium, while combining chunks ofthe data from two or more of the wireless communication units into eachof at least some of the frames.
 38. The method according to claim 37,wherein each of the frames comprises a plurality of slots ofsubstantially fixed length, and wherein combining the chunks comprisesinserting the chunks into respective slots together with chunk headersidentifying the respective frequency channels to which the chunksbelong.
 39. The method according to claim 37, wherein combining thechunks comprises transferring the chunks of the data from the two ormore of the wireless communication units in alternation into each of theat least some of the frames.
 40. The method according to claim 37,wherein conveying the data comprises conveying data packets, and whereinmultiplexing between the wireless communication units comprisesfragmenting the packets among the chunks.
 41. The method according toclaim 37, wherein multiplexing between the wireless communication unitscomprises inserting into the frames, in addition to the data, controlmessages for transmission between the access points and the hub.